Yanhui Sun

Theoretical study for OH radical-initiated atmospheric oxidation of ethyl acrylate

OH radical-initiated atmospheric oxidation of ethyl acrylate (ethyl 2-propenoate, EA) has been investigated by performing density functional theory (DFT) calculations. Optimizations of the reactants, intermediates, transition states and products were carried out at the MPWB1K/6-31+G(d,p) level. Single-point energy calculations were performed at the MPWB1K/6-311+G(3df,2p) level of theory. The detailed oxidation mechanism was presented and discussed. The results show that the OH addition is more energetically favorable than the H abstraction. Rice-Ramsperger-Kassel-Marcus (RRKM) theory was used to predict the rate constants over the possible atmospheric temperature range of 180-370 K. The Arrhenius expression adequately describes the total rate constant: k(EA+OH)=(1.71×10(-12))exp(805.42/T)cm(3) molecule(-1) s(-1). At 298 K, the atmospheric lifetime of ethyl acrylate determined by OH radicals is about 16.2h. In order to find out the effect of alkyl substitution on the reaction activity, rate constants for the reactions of methyl acrylate, methyl methacrylate and butyl acrylate with OH radicals were also discussed. Calculation results show that the reaction activity may increase with the increased electron-donating substitution for electrophilic addition reaction.

Mechanical and Kinetic Studies of the Formation of Polyhalogenated Dibenzo-p-dioxins from Hydroxylated Polybrominated Diphenyl Ethers and Chlorinated Derivatives

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs),which may be generated from PBDEs, are more toxic than their matrix and have been detected in organisms. In this article, we have focused on the gas phase formation of polyhalogenated dibenzo-p-dioxins from several OH-PBDEs and their chlorinated derivatives. All of the geometries and frequencies are calculated at the MPWB1K/6-31+G(d,p) level of theory. The single point energy is obtained at the MPWB1K/6-311+G(3df,2p) level. Rate constants of each step have been calculated over a wide range of 200-2000 K using the canonical variational transition state (CVT) theory with small curvature tunneling (SCT) contribution. The rate equations are shown through Arrhenius formulas. The presence of chlorine atoms increases the reaction barrier for the formation of major products.

Mechanistic and kinetic study on the ozonolysis of n-butyl vinyl ether, i-butyl vinyl ether and t-butyl vinyl ether

Density functional theory (DFT) and ab initio method are employed to elucidate the mechanisms for O(3)-initiated oxidation of n-butyl vinyl ether (n-BVE) and its isomers (i-BVE and t-BVE). For each BVE, the reactions proceed via O(3) cycloaddition resulting in the formation of primary ozonides (POZs) and then two self-decomposition pathways of POZs are followed. Major products are identified to be formaldehyde and butyl formates (CH(3)CH(2)CH(2)CH(2)OCHO for n-BVE, (CH(3))(2)CHCH(2)OCHO for i-BVE and (CH(3))(3)COCHO for t-BVE). The total and individual rate constants for main product channels have been calculated using the modified multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) approach. At 298 K and 101 kPa, the calculated total rate constants are 2.50×10(-16), 3.41×10(-16) and 4.17×10(-16) cm(3) molecule(-1) s(-1) for n-BVE+O(3), i-BVE+O(3) and t-BVE+O(3), respectively, which are in perfect agreement with experimental results. The total rate coefficients are almost pressure independent in the range of 0.001-101 kPa but obviously positive temperature dependent over the whole study temperature range (200-400 K). Also, the favorable reaction pathways have been determined through the estimation of branching ratios. Moreover, the influence of alkoxy group structure on the reactivity of vinyl ethers was examined.

Products, mechanism, and kinetics of OH radical-initiated oxidation degradation of 2,4,4′-trichlorobiphenyl in the atmosphere

To better understand the behavior, fate and oxidation products of polychlorinated biphenyls (PCBs) in the atmosphere, probing the atmospheric oxidation mechanism and kinetic properties of PCBs is of crucial importance. In this paper, taking 2,4,4′-trichlorobiphenyl (PCB28) as an example, the detailed atmospheric oxidation mechanism initiated by OH radicals was investigated by using quantum chemistry calculations. Studies show that the initial reaction is dominated by OH addition to form the PCB–OH adducts. In the atmosphere, the PCB–OH adducts could further react with O2 either by H-abstraction to form hydroxylated PCBs or by addition to generate peroxy radical intermediates, which subsequently cyclize to bicyclic radicals. Subsequent reactions include O2 addition, NO addition, NO2 elimination and benzene ring breaking. On the basis of the quantum chemical calculation, rate constants of the key elementary reactions at 298 K are calculated. The calculated overall rate constant of PCB28 with OH radicals is 1.25 × 10−12 cm3 per molecule per s with an atmospheric lifetime of 9.3 days. In addition, the effect of substituted chlorine atoms in PCBs to the reaction activity has been discussed.

OH radical-initiated oxidation degradation and atmospheric lifetime of N-ethylperfluorobutyramide in the presence of O2/NOx

The OH radical-initiated oxidation degradation of N-ethylperfluorobutyramide (EtFBA) in the presence of O2/NOx was investigated theoretically by using density functional theory (DFT). All possible pathways involved in the oxidation process were presented and discussed. The study shows that the H abstraction from the C(2)H(2) group in EtFBA is the most energetically favorable because of the lowest barrier and highest exothermicity. Canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution was used to predict the rate constants over the temperature range of 180-370 K. At 296 K, the calculated overall rate constant of EtFBA with OH radicals is 2.50 × 10(-12)cm(3)molecule(-1)s(-1). The atmospheric lifetime of EtFBA determined by OH radicals is short, about 4.6 days at 296K. However, the atmospheric lifetimes of its primary oxidation products, C3F7C(O)N(H)C(O)CH3, C3F7C(O)N(H)CH2CHO and C3F7C(O)NH2, are much longer, about 30-50 days. It demonstrates the possibility that the atmospheric oxidation degradation of polyfluorinated amides (PFAMs) contributes to the burden of observed perfluorinated pollutants in the Arctic region. This study reveals for the first time that the water molecule plays an important catalytic effect on several key elementary steps and promotes the degradation potential of EtFBA.

Adsorption and heterogeneous reactions of ClONO2 and N2O5 on/with NaCl aerosol

The adsorption and heterogeneous reactions of ClONO2 and N2O5 on the NaCl (100) surface have been investigated by performing density functional theory (DFT) calculations. Possible adsorption structures, energies and electronic properties of ClONO2 and N2O5 were studied by considering multiple possible adsorption sites without symmetry restriction. The most stable adsorption configurations are a vertical adsorption mode for ClONO2 and a horizontal adsorption mode for N2O5, with the adsorption energies of 14.81 and 13.67 kcal mol−1, respectively. Two possible reactions were proposed on the NaCl (100) surface. Adsorbed ClONO2 and N2O5 could react with HCl or hydrolyze on the NaCl surface to form HNO3 and other gaseous species (Cl2, HOCl and ClNO2). More importantly, ClONO2 and N2O5 can actively participate in the heterogeneous reaction with NaCl to generate Cl2 and ClNO2, which could be photolyzed to produce reactive chlorine atoms. The adsorbed H2O molecule plays an important role in the key elementary step and promotes the formation of the products. The present results rationalize previous experimental studies well and enrich our understanding of the source of halogen atoms in the marine boundary layer.